Always and Never

Pay attention. Many cases of experimental failure are repercussions resulting from inattention, or from Leaving the Lab while a reaction is underway.
Be prepared. When you start an experiment, the clock starts ticking. Make sure everything is ready beforehand.
Monitor regularly. Many students do not track their experiment closely enough.
When washing with aqueous bicarbonate, vent the separatory funnel often to prevent pressure buildup from evolving carbon dioxide.
Wash the organic layer with brine last.
Copy or print the entire journal article, not just the pages you need.
Allow compounds from the refrigerator to warm to RT before opening them.
Cospot your reaction on TLC.
Check to make sure your starting compounds are pure.
Store intermediates in the refrigerator or freezer.
Use tared flasks.
Stir reactions vigorously.
TLC a reaction as soon as it is begun.
Keep your rotovap bump trap clean.
Break the vacuum before turning off the vacuum source.
Put a flask under your separatory funnel before pouring anything into it.
Use a funnel.
Check to make sure your flask is clamped firmly before spinning the rotary evaporator.
Titrate n-BuLi prior to using.

See also: For a High Yield, Rules of Thumb

Leave your experiment: see Leaving the Lab and First Time Through.
Put your compound under high vacuum unless you know it won't evaporate.
Throw away any fractions from a column before you have an NMR of your product.
Throw away the aqueous layer of an extraction before you have an NMR of your product.
Pour neat compound- use a pipette.
Fill round-bottom flasks more than half full-(reactions, distillations, and rotoevaporation)
Heat a closed system without venting.

See also: Rookie Mistakes

Save a sample of the starting material when running a reaction for the first time.
Save a sample of the crude product when doing column chromatography.
Work up the reaction immediately after quenching it.
Notice if your reaction is becoming hot unexpectedly during any step, and cool the flask with an ice bath.
Maintain a homogeneous solution.
Maintain the correct temperature
Maintain stirring
If a solid crashes out of solvent at any point, filter it and analyze the solid and the filtrate.
If the compound is not volatile, put it on a high vacuum line for 10-20 minutes after rotoevaporation (and before NMR) to remove residual solvent.
Designate a notebook for literature search information. Record the date, the search parameters, what references were found, and which ones you looked up.